See the pic below ! and show work!

Well I don't know !
Let's check out the choices:

A).    3a - 4b = 21,  a=0, b=7

         3(0) - 4(7) = 21

             0  -  28  = 21    Nope.  That's not true.

B).  3a - 4b = 21,  a=-3,  b=2

     3(-3) - 4(-2) = 21

       -9    +  8  = 21

             -1    = 21      Nope.  That's not right.

C).  3a - 4b = 21,    a=-2,  b=-3

      3(-2) - 4(-3) = 21

         -6  + 12  = 21

                 6  =  21          Nope.  That's not it.

Only one choice left.
This one had better be it:

D).  3a - 4b = 21,    a=7,  b=0

       3(7) - 4(0) = 21

         21 -  0    = 21

               21  =  21            Yay !   That's it !
 

It is d because 3•7= 21
And 4•0= 0
21+0= 21

The solution to the system of equations (-2,2)